Produce Waste Treats Water Wastes

onionsGarlic and onion are ingredients found in numerous world cuisines, but now they may be a key ingredient in efficiently and affordably treating wastewater.

Indian researchers investigated (PDF) using discarded onion and garlic to mop up toxic compounds in industrial effluent and found it effective in removing heavy metals.

Scientists from the University School of Biotechnology at GGS Indraprastha University in Delhi, India, gathered onion and garlic wastes from market and food canning operations. The waste was dried at 50˚C for 24 hours, then screened to insure they had a relatively uniform substance.

To test the compound, they used industrial runoff from an electric motor factory in Delhi, which contained toxins including arsenic, iron, lead, nickel, and tin. They analyzed the allium wastes on both synthetic and industrial wastewater and found they were able to remove various toxic heavy metals.

Within 30 minutes of adding the dried onion-garlic mixture to water, it was reportedly able to concentrate almost 70% of the toxins in the wastewater.

Scientific American postulates that the approach may work “because onions and garlic contain a dietary fiber called inulin, which can bond with metal ions.”

Existing technologies typically used to remove heavy metals are expensive. The researchers note:

Conventional methods for heavy metal removal from aqueous solution and soil include chemical precipitation, electrolytic recovery, ion exchange/chelation, solvent extraction/liquid membrane separation and size exclusion processes. But these methods are often cost prohibitive having inadequate efficiencies at low metal concentrations. Moreover, the resulting sludge has to be concentrated and its disposal or recovery of metals from the sludge represents an enormous problem. The conventional technologies for effluent treatment are not economically feasible for small-scale industries that are prevalent in developing economies. Therefore, new technologies are required to reduce heavy metal concentrations to environmentally acceptable levels at affordable costs.

Biosorption is a property found in specific types of biomass that both binds and concentrates heavy metals in solutions. The biomass exhibiting these properties is typically an inactive, dead, microbial biomass. (Although live cells can also bind heavy metals, processes using them generally need added nutrients for them to work, which increases both biochemical and chemical oxygen demand in the treated water.) Scientists say the cell wall structure found in some algae, fungi, and bacteria is responsible for this phenomenon.

Scientists from McGill University in Montreal say they have successfully identified several types of biomass that are able to concentrate and bind heavy metals. These include microbes — some found in industrial fermentation by-products, such as Rhizopus or Bacillus subtilis — and various types of seaweed. The researchers say these types of biomass “can accumulate in excess of 25 percent of their dry weight in deposited heavy metals.” This includes lead, cadmium, uranium, copper, zinc, and chromium.

The Indian researchers say that water contamination by heavy metals is among the numerous problems triggered by the rapid growth of various industries in developing nations. This includes enterprises such as mining, fertilizer manufacturing, energy and fuel production, and leather processing. Effluent discharges from these types of facilities pose serious environmental and health problems.

The researchers say their approach to biosorption “appears industrially applicable and viable.” They add that this initial study “indicates great potential for the reduction of metal ions in wastewater. […] This may also provide an affordable, environmental friendly and low maintenance technology for small and medium scale industries in developing countries.”

They published their findings, “Biosorption of Heavy Metals by Utilising Onion and Garlic Wastes,” in the International Journal of Environment and Pollution.

Image by Alexandre Dulaunoy.